Process and device for determining the characteristics of a motor        &#39; vehicles built-in shock-absorbers

ABSTRACT

A process is proposed for determining the characteristics of a motor vehicle&#39;s built-in shock absorbers. The vehicle is driven onto a ramp support which is then quickly removed, causing the vehicle to drop onto a base whose distance below the ramp support corresponds to the residual rebound clearance associated with the particular vehicle design; the changes in the so-called wheel contact force when the car hits the base are measured. The process is characterized by the fact that the vibration characteristics of the body and one wheel at a time are measured, the measurement results, together with the measured values of wheel contact force, are fed into the known differential equations governing damped oscillations, and the characteristic data for the vehicle structure, namely, the body- and wheel masses, spring stiffness and damping constants, are calculated.

The invention relates to a process for determining the characteristicsof the shock-absorbers installed in a motor vehicle. According to thisprocess, the vehicle is run up a ramp and onto a support surface whichis then removed suddenly so that the vehicle drops onto a base. Thevertical distance between the base and the support surface correspondsto the residual wheel-rebound-clearance imposed by the design of thevehicle concerned. When the vehicle hits the base, the curve of thewheel-contact force exerted on the base is determined.

This process is described in EP Patent 2 269 81 [sic]. Using thisprocess to test the condition of the running gear of a motor vehicle, itis already possible, by suitable curve-analysis of the resultsrepresenting the wheel-force on the base, to produce, among otherthings, findings regarding the damping characteristics of the suspensionjoints of a motor vehicle, the quality of the shock-absorbers, and thehardness of the vehicle's springs. However, an evaluation of a vehicle'svibration-behaviour based on the graphs obtained when measuring thewheel-contact forces on the base provides no absolutecharacteristic-data values for the vehicle at the time of testing,because the effects of tyre-pressure, vehicle-loading, and the type oftyre fitted cannot be taken into account.

The aim of the present invention is to develop a process whereby thecharacteristics of a shock-absorber fitted to a motor vehicle can bedetermined without having to detach the shock-absorber, irrespective ofsuch variables as the tyre-pressure, type of tyre, vehicle-load, etc. Inother words, the aim of the invention is to provide a process wherebyall those forces which, in the known process, codetermine thevibration-behaviour of the vehicle once it hits the base, can beeliminated; and whereby the actual values for the damping constants andspring-stiffness--i.e. the current condition of the shock-absorber--canbe computed from the measured values.

This aim is achieved as follows, according to the invention:

the vibration-behaviour of the body and of one wheel at a time ismeasured,

the measurement results thus obtained, together with the measured valuesfor the wheel contact force, are introduced into the known differentialequations for damped vibrations, and

the characteristic data for the running gear of a motor vehicle arecomputed--said characteristic data being: the body and wheel masses,spring-stiffness, and damping constants.

The advantage of the process according to the invention, compared withthe prior-art processes, is that the result obtained with the processaccording to the invention is independent of the tyre-pressure, type oftyre, or loading of the tested vehicle, and gives the actual valuesvalid at the time of testing.

By determining the overall vibration-behaviour of the body and of onewheel in each case, it is possible to obtain not only a damping-value,but also the damping-characteristic of the shock-absorber, divided intotension and compression regions. A further advantage of the new processis that the total testing-time is very short, and both wheels of awheel-axis can be tested simultaneously after the shock-producingimpact. As well as the characteristic curve for the shock-absorber, thestiffness of the vehicle's springs and the effective masses can also bedetermined.

The possibility of obtaining all the data needed to evaluate thecondition of the vehicle by means of a single, brief measuringoperation, makes the process suitable for:

series-testing e.g. by regulatory authorities, and by the automobileindustry (for end-of-production-line checking and running-gearanalysis); and

workshop diagnosis of faults in motor vehicles already in service,

because the process of the invention enables the vehicle's actual valuesto be compared with the required values.

Preferably the vibration-behaviour of the vehicle-body and of each wheelof the wheel-axis being tested can be determined by measuring thedisplacement of the translational vibrations of the body and wheel, foreach side of the wheel-axis being tested; such displacement-measurementis performed either by individually determining the vibration-amplitudesof the body relative to the base and the vibration-amplitudes of thewheel relative to the base, or by determining the amplitude-differencebetween these two vibrations. It may be preferred, however, to determinethe curve of the vibration by measuring the velocity of the motion ofthe body and wheel, or by measuring the acceleration during the motionof the body and wheel, after they have hit the bases.

An apparatus suitable for performing these measurements comprises:

not only

the measuring device (known in the art) for measuring the characteristiccurve of the wheel contact force on the base,

the base, with an up-ramp and removable vehicle support surface arrangedabove this base at a height corresponding to the residual reboundclearance of the vehicle, and

a measuring device for determining the curve of the wheel contact forceacting on this base after the vehicle has dropped onto it, as a functionof time,

but also

a further measuring system suitable for determining the curve of thevibration-behaviour of the body and wheel as a function of time.

A measuring system of this type can comprise an inductive pick-up, avelocity-measuring means, an acceleration-measuring means, or a [laser];or it can be equipped with an image-recording system.

Details of the measurement-process, the design of suitable devices forits implementation, and the method of evaluating themeasurement-results, will emerge from the following description ofexamples of the implementation of the invention, and from the drawings,in which:

FIG. 1 is a diagrammatic representation of a test stand, with which animage-recording system is being used;

FIG. 2 is an equivalence-model representing the forces occurring in amoving motor vehicle;

FIG. 3 is a plot of the vibration frequency of the wheel contact force,as obtained with the test set-up shown in FIG. 1;

FIG. 4 is a plot of the values obtained experimentally when measuringthe vibrational displacement of the body and wheel, compared with thecalculated values (test set-up according to FIG. 1);

FIG. 5 is a plot of the characteristic curves obtained with repeatedtests (test set-up according to FIG. 1);

FIG. 6 shows characteristic curves of damping obtained for a vehiclewith shock-absorbers of differing quality (test set-up according to FIG.1);

FIG. 7a shows a target to be applied to the wheel of the motor vehicle;

FIG. 7b shows a target to be applied to the body of the motor vehicle;and

FIG. 7c shows a target to be applied to the vehicle's body, enabling themovement of the body in both the vertical and the horizontal directionsto be determined.

A test stand for implementing the novel testing process comprises a ramp1, with two trapdoor-like drop panels 2a, 2b at the top. The distancebetween these two drop panels 2a, 2b is approximately the same as thatbetween the two wheels of a wheel-axis of a motor vehicle. The motorvehicle that is to be tested is driven up an inclined approach surface3, so that the two wheels 4 of either its front or its rear wheel-axiscome to rest on the drop panels 2a, 2b. The upper surface of the droppanels 2a, 2b is approximately 50 mm above a base 5, which is designedas a weighing unit 6, e.g. a strain gauge or the like. The selection ofthe distance 7 between the drop panels 2a, 2b and the base is determinedby the residual rebound clearance (i.e. the distance between the wheelaxle and a resilient stop) imposed by the design of the vehicle. This isso as to prevent the wheel--during its free fall after the dropping-awayof the panels 2a, 2b--from being arrested by the resilient stops beforereaching the base 5, which would lead to false vibration-results.

The weighing unit 6 serves to measure, as a function of time, the forceon the base 5 resulting from the vibration-behaviour of the vehicleafter the wheels 4 have hit the base 5.

This force will be referred to below as the "wheel contact force". Itconstitutes an essential component for the subsequent numericaldetermination of the characteristics of the shock-absorber.

A displacement-measuring system 8a, 8b is provided beside, but at somedistance from, each of the front wheels 4 of the motor vehicle restingon the test stand. In FIG. 1, the displacement-measuring system 8a, 8bis in the form of an image-recording system, which makes it possible torecord the curve of the vibrations of the body 9 and wheel 4 withoutcoming in contact with them. The curve of the vibration amplitudes ofthe body 9 and wheel 4 after falling through the drop distance 7 formsthe second component necessary for the numerical determination of thecharacteristics of the shock-absorber. For this purpose, targets areapplied to the wheel and vehicle-body, and the vibration processes aredetermined by means of an image-processing system. The target for thebody consists of a bar pattern, and the target for recording thewheel-vibrations consists of a rotationally-symmetrical pattern ofalternate black and white rings.

However, when the motor vehicle falls onto the base, body-vibrations notonly occur in the vertical direction, but also, at the time of impact,small body-vibrations are detected in the vehicle's longitudinaldirection as well. Because the measuring-method used here givesextremely accurate measurements of vibration-behaviour, these horizontalvibration movements should also not be ignored. In order to detect them,however, it has proved advantageous to modify the target bar patternknown in the art, which consists of horizontal black and white bars.This pattern is altered in such a way that the back-and-forth movementof the vehicle's body in the vehicle's line of travel can be determined.This modification of the target from one used solely for verticalvibrational movements to the one required here for more accuratemeasurements is shown in FIG. 7c. FIG. 7a shows the target 11 consistingof concentric black and white rings, which is applied to the wheel. FIG.7b shows a target 12 which is applied to the body, and is used in caseswhere only the vertical vibrations of the body are to be measured. Thistarget 12 consists of parallel horizontal black and white bars. FIG. 7cshows a target that makes it possible to determine the movement of thebody not only in the vertical direction but also in the horizontaldirection, i.e. the direction of travel. The black and white bars, whichare parallel in FIG. 7b, are inclined at an angle to horizontal in oneregion of the target shown in FIG. 7c.

The evaluation of the test results will now be described.

It should first be mentioned that the computation method described belowis based on a simplified model of a motor vehicle, shown in FIG. 2. Inthis model: m₁ is the mass of the wheel 4 and wheel axle; m₂ is the massof the body 9, including any load carried in the vehicle being tested;the coefficient c₁ designates the spring constant, which is given by thetyre-elasticity; and this elasticity experiences a damping designated byd₁.

The body 9 is connected to the wheel 4 by a spring 10 whose springconstant is c₂. The vibration of the spring 10 is damped by ashock-absorber whose damping constant is d₂.

The roadway 12 indicated in FIG. 2 has an uneven surface. Its maximumunevennesses correspond to the residual rebound clearance, i.e. the dropdistance 7 of the test stand.

The model shown corresponds to a vibrating-system in which the masses m₂and m₁ are connected to each other by two series-connected dampedsprings. The equations of motion of a two-mass vibrating-system of thistype are as follows, assuming that the spring forces and damping forcesare linear functions of the relative excursion and relative velocityrespectively:

    m.sub.2 ·y.sub.2 (t)+d.sub.2 [y.sub.2 (t)-y(t)]+c.sub.2 ·[y.sub.2 (t)-y.sub.1 (t)]=0                     1.

    m.sub.2 ·y.sub.2 (t)+d.sub.2 ·[y.sub.1 (t)-y.sub.2 (t)]+c.sub.2 ·[y.sub.1 (t)-y.sub.2 (t)]+

    d.sub.1 ·[y.sub.1 (t)-y.sub.0 (t)]+c.sub.1· [y.sub.1 (t)-y.sub.0 (t)]=0                                        2.

in which

m₁ =means the moving masses of the wheel, wheel-axle, andwheel-suspension;

m₂ =is the moving portion of the mass of the vehicle body;

c₁ =tyre-elasticity;

c₂ =stiffness of vehicle-spring;

d₁ =tyre damping; and

d₂ =damping constant of a shock-absorber installed in the vehicle.

y₀ (t)=the translational vibration of the base, as a function of time;

y₀ (t)=the velocity of the base, as a function of time;

y₁ (t)=the translational vibration of the wheel, as a function of time;

y₁ (t)=the wheel velocity, as a function of time;

y₁ (t)=the wheel acceleration, as a function of time;

y₂ (t)=the translational vibrations of the body, as a function of time;

y₂ (t)=the velocity of the body, as a function of time; and

y₂ (t)=the acceleration of the body, as a function of time.

The two equations of motion can be combined into a differentialequation, by introducing the difference between motion of the wheel andthat of the body, in place of the vibration-amplitudes thereof, thusobtaining:

    m.sub.1 ·x(t)+d.sub.2 ·A·x(t)+c.sub.2 ·A·x(t)=f(t)                            3.

where:

a) x(t)=y₂ (t)-y₁ (t), with Y₂, and y₁ (t) being measured values, namelythe curves of displacement of the body and the wheel over time (thedisplacement-difference being computed!);

b) ##EQU1## are measured values for the wheel and body c) f(t)=-d₁ ·[y₀(t)-y₁ (t)]-c₁ ·[y₀ (t)-y₁ (t)] is likewise a measured quantity, namelythe curve of the wheel contact forces (dynamic wheel load) over time.

The Fourier spectrum of the wheel contact force over frequency is knownin the art. It is plotted again in FIG. 3.

In the chart, the frequency is shown on the horizontal axis, with alogarithmic scale running from 10⁻¹ to 10⁻³ Hz. The wheel contact forceis shown as f(t) on the vertical scale, which runs from 10⁰ to 10^(4N).At approximately 1.25 Hz, a first maximum occurs, corresponding to thevibration of the body. At approximately 9.8 Hz, the second maximumoccurs, corresponding to the wheel frequency.

Before the above-described method of determining the individualparameters was decided on, testing was performed to determine whetherthe idealized model on which the method is based was in fact applicable,because e.g. vibrations are also set up in the untested wheel-axis whenthe vehicle is dropped onto the base.

Therefore the test vehicles were first equipped with a total of 8measuring sensors, which were attached to the body- and wheel-axes atintervals.

The results showed, however, that scarcely any rotational movementsoccur around the longitudinal axis of the vehicle, and therefore littlevibration energy is transmitted from one side of the vehicle to theother. Also, the untested wheel-axis has little effect on the testedwheel-axis; little vibration energy is transmitted to the latter.

Accordingly it appears possible to consider the left and right sides ofthe vehicle separately from each other in the region of the testedwheel-axis, and without regard to the wheel-axis that is not beingtested. In other words, the above-described equivalence-model of atwo-mass vibrating system can be applied without fear of falsifying theresults.

Using the measuring system shown in FIG. 1, it is therefore possible tomeasure the curve of the wheel contact forces f(t) over time by means ofthe weighing device 6, and to determine x(t) by means of thedisplacement-measuring system 8a and 8b. Suitable programming of acomputer to which the large number of measurement signals from theweighing unit 6 and the displacement-measuring system 8a, 8b were fed,enabled the desired model parameters m, d, and c to be computedaccording to the least-squares method of estimating true values frommeasured values subject to error.

In FIG. 4, the computed and measured values are plotted against time inseconds, using the inductive measuring method for the difference indisplacements. It can be seen clearly that the two curves match eachother well.

This shows that the chosen computation-method is suitable.

After the model parameters m, c, and d have been determined, however,the effective values for the wheel mass m₁, body mass m₂,spring-stiffness c₂, and damping constants d₂ can now be determined,because the static part of the wheel contact force fst provides arelation between these values: f_(st) =m₁ +m₂ +g, where g is theacceleration due to gravity=9.8065 m/[s² ]. The following relations ofthe chosen model-parameters (m, c, and d) to m₁, m₂, d₂, and c₂ alsoapply:

    m.sub.1 =m

    m.sub.2 =f.sub.st /g-m.sub.1

    d.sub.2 =d·m.sub.2 ·g/f.sub.st

    c.sub.2 =c·m.sub.2 ·g/f.sub.st

The above concepts and relations make it possible to produce asymmetrical (tension:compression=1:1) linear damping characteristic withreference to the wheel contact point for a shock-absorber installed in amotor vehicle. In practice, however, the shock-absorbers used generallyhave an asymmetrical characteristic curve.

Here, the required result will be obtained either by having a regionallinear equation-arrangement for the damping force, or by having aquadratic arrangement with linear and quadratic terms of differentmagnitudes in the tension and compression regions.

FIG. 5 shows the test results from five tests on a Mercedes motorvehicle. As can be seen from the plots in FIG. 5, there were scarcelyany differences between the individual results. The computed values forthe parameters show deviations from the measured values of between 1 and2%.

The test set-up for these last-mentioned measurements corresponded tothat of FIG. 1. An inductive measuring system was used.

The measuring method used was: the determination of the difference indisplacements. The measured values from the displacement-measurementsystem and the weighing unit were fed into a computer, whose softwarewas programmed according to the mathematical vibration equations.

The same test set-up can also be used for determining the individualamplitudes of the body and wheels by means of thedisplacement-measurement system; but this in no way changes theprinciple of the computing method.

It is however also possible, and in certain cases advantageous, tocompute the desired values by using a measuring system in which thevelocity-differences between the body and the wheel-masses aredetermined, instead of by determining the difference in displacements.Similarly, a measuring method can also be chosen in which theacceleration-differences between said measured quantities aredetermined. Apparatuses suitable for this purpose are known to personsskilled in the art, and therefore the test set-up for these measuringmethods need not be explained in detail here.

FIG. 6 illustrates the efficacy of the method according to theinvention.

This graph shows the characteristic curves of damping for different testseries performed on the same test vehicle. For determining thesecharacteristic curves, the test vehicle was first tested withoutmodification. Then the new right rear shock-absorber was replaced by a40% shock-absorber and the vehicle was tested again.

Consideration of the damping curves shows that the residual damperperformance values are of different magnitudes in the tension andcompression regions. Whereas the residual damper performance in thecompression region is approximately 13%, it is 53% in the compressionregion.

By performing such an evaluation, it may be possible to determine thetype of damage to the shock-absorber on the basis of the change in thecharacteristic curve.

From what has been said above, it can clearly be seen that theshock-absorber testing method according to the invention sets newstandards for evaluating the characteristic curves representing thecondition of a motor vehicle.

The claims defining the invention are as follows:
 1. A process fordetermining a response to a force shock-absorbers installed in a motorvehicle, comprising:(a) positioning the motor vehicle on a supportsurface of a ramp, (b) removing suddenly the support surface so that themotor vehicle drops a predetermined distance from the support surfaceonto a base, said predetermined distance corresponding to apredetermined residual rebound clearance defined by the vehicle's designspecifications, (c) measuring wheel contact forces exerted on the baseas a function of time when the wheels of the motor vehicle hit the base,(d) measuring at least one of a displacement, a velocity, and anacceleration of the vehicle body and of at least one wheel as a functionof time, andcomputing using results of said measuring steps (c) and (d)values for at least one of a vehicle body mass, a wheel and wheel axlemass, a vehicle spring-stiffness, a tire spring constant, a tire dampingconstant, and a shock absorber damping constant.
 2. A process as claimedin claim 1, wherein said measuring step (d) comprises measuring adisplacement of the vehicle body and at least one wheel.
 3. A process asclaimed in claim 2 wherein said displacement is measured by (i)determining individually the vibration-amplitudes of the body relativeto the base and those of the wheel relative to the base, or (ii) adifference in amplitude between these two vibration-amplitudes.
 4. Aprocess as claimed in claim 1, wherein said measuring step (d) comprisesmeasuring a velocity of the the vehicle body and said at least onewheel.
 5. A method as claimed in claim 1, wherein said measuring step(d) comprises measuring an acceleration of the vehicle body and said atleast one wheel including at least a period of time when the wheelcontacts the base following the step of removing suddenly said supportsurface.
 6. A device for implementing the process as claimed in claim 1,comprising:a ramp and support-surface for the motor vehicle, saidsupport-surface being arranged removably above a base at a verticaldistance therefrom corresponding approximately to the motor-vehicle'sresidual rebound clearance, and a measuring device for determining thewheel contact force acting on the base as a function of time after themotor-vehicle has been dropped onto said base, wherein a measuringsystem is provided to measure at least one of a displacement, avelocity, and an acceleration of the vehicle body and of at least onewheel as a function of time.
 7. A device for implementing the process asclaimed in claim 6, wherein said measuring system comprises an inductivepick-up.
 8. A device for implementing the process as claimed in claim 7,wherein said measuring system comprises a means for measuring velocity.9. A device for implementing the process as claimed in claim 6, whereinsaid measuring system comprises a means for measuring acceleration. 10.A device for implementing the process as claimed in claim 6, whereinsaid measuring system comprises a laser.
 11. A device for implementingthe process as claimed in claim 6, wherein said measuring systemcomprises an image-recording system.
 12. A device for implementing theprocess as claimed in claim 6, further comprising a computer, whereinmeasured values of said wheel contact forces exerted on the base whenthe wheels of the motor vehicle hit the base and measured values of saidat least one of a displacement, a velocity, and an acceleration of thevehicle body and of at least one wheel are input into the computer,which is programmed to output values for at least one of a vehicle bodymass, a wheel and wheel axle mass, a vehicle spring-stiffness, a tirespring constant, a tire damping constant, and a shock absorber dampingconstant.